<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>CSS</title>
    <style>
        /* 
        id选择器只能选择一个;
        因为将来使用js选择dom元素会只操作1个！
         */
        #red {
            color: red;
        }
    </style>
</head>

<body>
    <p>段落1</p>
    <p id="red">段落2</p>
    <p>段落3</p>
    <p>段落4</p>
    <p>段落5</p>
    <p>段落6</p>
</body>

</html>